1. Field of the Invention
This invention pertains to lasers. More particularly, it pertains to methods and devices for compressing the length of electromagnetic pulses generated by lasers.
2. Description of the Prior Art
The short, ultraviolet wave-length and the high efficiency of the rare gase halide excimer lasers make them very promising candidates as drivers of a thermonuclear fusion reactor (see e.g. Physics Today, May 1978, page 39). However, the pulse length of these lasers, which is on the order of 100 nanoseconds, is too long for this application. For this reason, methods and devices are being developed to compress the pulses produced by these lasers to pulse lengths on the order of 1 to 10 nanoseconds. One device being developed utilizes backward Raman scattering.
U.S. Pat. No. 3,515,897, titled "Stimulated Raman Parametric Amplifier", describes in some detail the manner in which the backward-wave, Raman amplifier operates to transform the energy contained in a relatively lengthy pulse of pump power (the primary beam) into a relatively short burst of electromagnetic radiation at the Stokes-Raman frequency (a frequency displaced from the frequency of the pump beam by the Raman shift of the scattering medium). The backward wave Raman amplifiers in the prior art utilize beams of pump power and beams of Stokes-Raman pulses in the scattering medium which have a uniform cross-sectional area and shape throughout the scattering medium. A few of the prior art devices, e.g. U.S. Pat. No. 3,515,897, in some instances provide for focusing of the pump beam within the scattering media in order to initiate the Stokes-Raman pulse at the focal point. These focusing arrangements, however, are not oriented so as to be coordinated with the build-up of the energy density in the Stokes-Raman pulse as it progresses through the amplifier.